It depends on what species they are. Most likely you have Pomacea bridgesii, which are the "mystery snails" you will find at most chain pet stores like Petsmart and Petco. They only eat decaying plant matter, so if you see them nibbling it's likely on dying leaves or bits that have fallen off. In that regard they're a great cleanup crew for planted tanks. If you see them nibbling excessively, though, that's usually a sign that they're hungry. What have you been feeding them?
Sometimes Pomacea canaliculata is sold as a "gold mystery snail" as well, though, and in that case you should be seeing entire plants decimated in a matter of days. They're also pretty unusual in the pet trade, too. So as long as you're not seeing giant holes or ravaged plants, you have P. bridgesii and therefore nothing to worry about.